The present invention relates to the field of display structures and display architecture. In particular, this invention relates to the architecture of passive liquid crystal display (LCD) flat panels.
The number of applications for display technologies is rapidly increasing. However, display technologies have been generally employed in relatively small devices, such as, notebooks, computers, cameras, telephones, projection displays and direct-view flat panel televisions. For example, recent digital cameras employ flat panels as viewfinders and play back displays. Commonly, super twisted nematic (STN) displays are employed in such applications. On the other hand, more expensive thin film transistor (TFT) LCDs have applications in notebook computers and flat panel monitors.
The television industry is beginning to use flat panel displays and supplant older unwieldy cathode ray tube (CRT) technologies. For example, LCDs are becoming common in hand-held televisions. LCDs also provide some digital performance enhancements over older cathode ray tube (CRT) technologies.
The television market has been dominated by the CRT for the past few decades. As a result, the mature CRT manufacturing industry has been able to significantly lower the production cost of CRTs. Nevertheless, the market price for smaller televisions has eroded so that the cost to build a small CRT-based television is very close to the selling price. Therefore, there is little margin in the manufacturing cost to permit lowering the price to compete with lower cost alternatives. Consequently, a cost-driven market window for TV displays is apparent. Yet, current LCD architectures are too costly to exploit this potential opportunity and suffer from acceptance-limiting performance shortcomings. For example, it is too costly to build television size LCD displays, and the contrast ratio of most LCD displays has yet to rival CRTs.
Non-CRT based displays generally have limited viewing angle, poor contrast, low brightness, and high manufacturing costs. Numerous attempts have been made to overcome these drawbacks by devising different architectures and employing various materals.
For example, the electro-luminescent display showed early promise as a competitor to the CRT, but these displays were never able to match the color requirements or price goals for the television market. Currently, plasma displays may be the best contender to replace CRTs in the television market. Other than cost, plasma displays have all the attributes required for the television industry. Because of price, plasma displays are, targeted toward the commercial market and the high definition television consumer. Facilities to build these devices are very specialized and expensive.
Field emission displays have also been tried in television applications. Field emission displays are essentially flat CRTs that replace a single electron emitter (gun) with millions of tiny emitters. Theoretically, these displays should have exactly the same performance as CRT""s. However, manufactured field emission displays have fallen short of expected performance and cost goals. Today, they have limited commercial and military sales in applications requiring ruggedness and low color definition such as instrumentation.
Currently, the premiere LCD flat panel is the thin film transistor (TFT) display. TFT displays employ one transistor (switch) for each sub pixel of the display (three sub-pixels red, green and blue make one pixel). Consequently, TFT displays are relatively expensive. A typical XGA flat panel has, for example, 2,359,296 transistors. In addition, each transistor must be functional. Development of production equipment and technology to reach significant yields has resulted in high fabrication costs. The typical factory investment is greater than several hundred million dollars. Additionally, millions of dollars every year are spent in research and development, increasing the costs by a significant amount. TFT displays are used in notebook computers, small hand-held televisions and monitor displays, and other similar devises.
In contrast to TFT displays, the super twisted nematic (STN) display is a much simpler device and the lowest cost LCD display technology available today. In a STN display, the pixels are formed by an orthogonal grid of transparent conductors placed on adjacent plates or substrates. The performance of these displays does not equal TFT""s, but the cost is significantly less. Passive displays are used in common devices such as watches, gauges and games.
STN LCDs suffer from some performance shortcomings. For example, on existing STN LCD""s, two polarizers are employed on each display. The polarizers reduce the light passing through the LCD by approximately 50%. Additionally, the liquid crystal display media must twist a certain amount to align the light between the polarizers. Consequently, the spacing between the top and bottom plates or substrates, the cell gap, is extremely critical. To maintain uniform spacing, the top and bottom plates are polished. Thousands of precision spacers are then sprayed onto one of the plates. These spacers maintain exact separation of the plates so that the liquid crystals twist no more than required to align the top and bottom polarizers. If the gap is too large or too small, the crystal xe2x80x9cover twistsxe2x80x9d or xe2x80x9cunder twists.xe2x80x9d Any variation in thickness or twist causes distorted images.
The viewing angle of a typical LCD is affected by the orientation of the light passing through the polarizers. Optimum viewing is obtained if the orientation of the light is toward the observer. The result is a brighter display with a wider viewing angle. The light is oriented at a specific angle based on the polarizer orientation. To control light orientation, LCD manufacturers purposely modify the display elements (pixels) by a method referred to as xe2x80x9crubbing.xe2x80x9d Rubbing causes the LCD material to polarize or orient in the direction of rubbing. To widen the viewing angle, some manufactures modify the LCD material in the display element with a special rubbing technique by rubbing one half of the display element to spread some of the light out the right side of the display element and rubbing the left side of the display element to have some of the light spread out of the left side of the display element. In this way, the light is directed both left and right toward the observer. Premium designs divide the display element into four differentially rubbed sections to direct the light vertically out the top and the bottom of the display element as well as left and right. Such LCDs have greatly improved viewing angles, but at the cost of less overall brightness due to light spreading in four directions.
Viewing angle can also be improved by making the display thinner. This reduces parallax effects by shortening the distance from the light source to the xe2x80x9clensxe2x80x9d (i.e., the LCD element). LCDs take considerable time to turn completely ON or OFF. Consequently, in a television application where relatively high frame rates are required, LCDs generally do not have sufficient time to turn completely ON or OFF. This is manifested in low contrast. Contrast is a major problem for non-emissive devices such as LCD""s which control a light passing from back to front. Therefore, because of addressing time limitations, a compromise is made between light OFF and light ON.
Generally, matrix address displays have activating drivers that provide data on one set of data electrodes and another set of drivers for scanning electrodes. In such displays, the electrical connections to the scanning electrodes remain connected to the driver output even while the scanning electrode is not selected (not driven). Thus, display elements associated with the data electrode may pick up charges from addressed display elements, but not the addressed row. This charge spill-over, sometimes called xe2x80x9ccrosstalk,xe2x80x9d happens when the data electrode provides unintended charge to the display elements associated with adjacent selector electrode(s). Poor contrast ratio is the result. Typically, the vertical electrodes contain data and bear different voltage levels applied to them which control the intensity of the LCD display element. Every time the voltage is turned ON for a particular display element, a small amount of charge leaks to adjacent display elements. Some of these proximal display elements get partially turned ON, leading to the appearance of unintended voltage on these adjacent display elements resulting in a poor contrast ratio.
Several attempts have been made to solve the problem of poor contrast ratio in LCD display systems, in particular passive matrix displays, but most prior art solutions have achieved limited success. The flat panel liquid crystal color displays of the prior art have had certain features that have compromised their acceptability.
By way of example, U.S. Pat. No. 3,765,011 to Sawyer et al. purportedly uses capacitors and switches. U.S. Pat. No. 4,516,053 to Amino describes a flat panel display apparatus with two insulating plates separated by striped barriers. In Amino, however, a brilliant display is purportedly achieved by reducing the width of transparent electrodes at the observing side of the display. U.S. Pat. No. 4,832,457 to Saitoh discloses a multi-panel liquid display device. In order to provide a large displayable area, a plurality of liquid crystal display panels are combined together to form a display device to overcome and mitigate the effects of a lattice-like blank problem, which is inevitably formed at the joint between the display panels. In U.S. Pat. No. 5,237,437 to Rupp, a high contrast, wide viewing angle liquid crystal display is purportedly disclosed. However, this display architecture entails a method of fabrication that manipulates the thickness of sub-pixel color display elements and liquid crystal thickness to control optical transmission. Other known methods in the prior art employ polarizers and color filters to improve viewing angle and brightness. For example, the crossed polarizer, twisted nematic type of liquid crystal display has a transmission through the liquid crystal display element that is uninhibited for zero applied voltage. This liquid crystal display configuration is referred to as a normally white display and is used in many display applications such as in watches and calculators. Generally, linear polarizers are oriented in a mutually parallel configuration. This provides a display with no optical transmission (i.e., black) when the liquid crystal media is not activated. In such displays, the optical transmission increases with applied voltage.
In order to compete in the large display television market, the performance of the display cannot be less than the performance of projection television or CRT displays. Poor contrast ratio, high drive voltage, low brightness, limited viewing angle and the cost of current passive LCD panels have all prevented large screen passive LCD television products from entering the non-commercial television market.
It is, therefore, an object of the invention to improve the contrast ratio of passive LCD displays.
Another object of the invention is to lower the drive voltage requirement for passive LCD displays.
A further object of the invention is to produce flat panel displays efficiently, while decreasing the cost of manufacturing.
Yet another object of the invention is to improve the viewing angle and brightness of passive LCDs.
Still another object of the invention is to produce a display panel architecture with a thin design and light weight that minimizes or eliminates handling and shipping problems exhibited by large bulky projection and large screen CRT products yet provides an efficient and high performance display.
It is another object of the present invention to provide a display architecture which can, if desired, be fabricated with either a compact array of picture elements, thus rendering it suited to use in a projection display system or with a large number of picture elements to provide a high resolution display.
The display structure of the present invention includes parallel opposing major substrates. A predetermined pattern of transparent conducting, electrodes are placed on inner surfaces of the optically transparent substrates. Column electrodes are placed on one substrate while row electrodes are placed on the other substrate. An intermediate double-sided, optically transparent intermediator substrate having transparent conducting material textured on both sides is interposed between the two electrode bearing substrates. LCD material is disposed between the column electrode substrate and the intermediator substrate. Similarly, LCD material is disposed between the row electrode substrate and the intermediator substrate.
In a preferred embodiment of the present invention, a glass substrate is textured with indium tin oxide (ITO) row electrodes and another glass substrate is textured with ITO column electrodes. A double-sided ITO-textured intermediator substrate is disposed between the two electrode substrates as a common plane. The glass column substrate and row substrate are sandwiched together with the intermediator common plane between. A variety of timing devices may be employed to achieve timing synchronization for the display. As those skilled in the art recognize, well known timing structures, devices and techniques of many different types will satisfy the requirements presented by the timing synchronization needed to address the display of the present invention.
Typically, activating signals of opposite polarities are applied preferably in the alternate frame periods across the LCD material to minimize or reduce the rate of deterioration of the LCD material generally caused by repetitive twisting of liquid crystals. The electro-optical characteristics of an image location, e.g. whether it will appear dark, bright or an intermediate shade, is determined by the orientation of the liquid crystal molecules within that image location under the influence of an electric field. For example, in root mean square (RMS) responding displays, changes in alignment of liquid crystal molecules under electric field excitation also change the optical characteristics of the LCD material. The direction of orientation can be altered by the application of an electric field across the image location which field induces a dielectric torque on the molecules that is proportional to the square of the applied electric field. The applied electric field can be either a DC field or an AC field.
The common plane can either serve as a plane of fixed reference potential or it can be switched between different levels of potential, preferably between ground potential and a positive voltage level. In a first addressing method, with the common plane at a fixed reference, the potential difference across the LCD material can be driven between a positive and negative voltage level to generate an RMS (root mean square) voltage. The fixed reference potential can optionally be at ground potential.
In the second addressing method, the common plane is devised to be switchable between a fixed reference potential and another potential. A voltage signal of one polarity is sufficient to generate an RMS voltage across a display element in the second addressing method. Because rather than requiring a positive and negative polarity voltage signal to generate an RMS, the switchable polarity attribute of the common plane allows it to switch between the ground and a positive polarity which provides a similar effect of switching between a positive and negative polarity voltage signal. Consequently, the design of the driving system of the display structure is simplified.
Those skilled in the art will recognize that many widely used methods for addressing passive matrix LCDs based on the techniques described by P. Alt and P. Pleshko in numerous sources that known by those skilled in the art may be employed in the above explained first addressing method. Another traditional method of addressing passive matrix liquid crystal displays as described in U.S. Pat. No. 5,420,604 to Scheffer et al. may be employed in the first addressing method case when the common plane is at a fixed reference potential.
However, in the above described second addressing method, since the common plane is devised to be switchable between a fixed reference potential and another potential, the traditional addressing methods may be accordingly readily modified. As will be appreciated by those skilled in the art, in accordance with the above mentioned second addressing method, a hardware implementation of an addressing system may suitably be devised with a simple modification of the controller to provide the alternate polarity activating signals to the common plane structure, preferably in alternate frame periods. This invention lowers the drive voltage required to meet the RMS turn ON voltage of the display element. The common plane divides the display structure thickness, thereby requiring significantly lower drive voltages to produce a given contrast ratio. In the second addressing method, the maxi mum drive voltage on the row electrodes and the column electrodes can be approximately equal. Moreover, preferably drive voltage dynamic range may be of single polarity. For example, the drive voltage does not have to swing between the same magnitudes of positive or negative polarity, rather preferably it can go between some positive potential and ground. Consequently, drivers can be operable with a single polarity voltage. They may even drive at identical voltage levels, which is not so in existing designs of display structures.
The drive voltage may be further lowered by having a comb-shaped design of the row electrode bearing substrate column electrode bearing substrate and the common plane forming a plurality of levels. Based on the number of these levels in the design, the drive voltage may be further lowered while keeping the same overall thickness of the display structure. However, as those skilled in the art will appreciate from this disclosure, other combinations of the structures and principles shown here are adaptable to the present invention to lower the drive voltage.
The poor contrast ratio performance of passive LCD display structures is significantly improved by the intermediate common plane-based display architecture of the present invention. Typically, crosstalk results due to charge spill-over effects, while a column electrode is driven, contributing to the deterioration of visual contrast of a display. A variety of signal regulation schemes may be employed to achieve reasonable electrode potentials to enhance the contrast ratio. The intermediate common plane provides a more complete ON or OFF transmittance, as well as gray scale-based state for each display pixel, thereby significantly improving the contrast ratio.
In a preferred embodiment of the present invention, image locations are formed by the overlap of the column and row electrodes. An image location is formed at the overlap of corresponding row and column electrodes and is comprised of row and column display elements. The row display element of an image location is formed by the overlap of the corresponding row electrode with the common plane structure, and the column display element of the image location is formed by the overlap of the corresponding column electrode with the common plane structure. Column drivers are selectively connected to a set of the column electrodes to apply a set of activating column signals to the set of column electrodes. Similarly, row drivers selectively connected to the plural row electrodes to selectively apply activating row signals to selected column electrodes. The row driver is selectively connected to a row electrode to apply an activating row signal to the corresponding row electrode.
In an alternate preferred embodiment of the present invention, a wire grid mesh can be employed as a common plane. In another preferred embodiment a film of light absorbing or directing material is disposed adjacent to and coextensive with the two electrode substrates and the common plane. For example, in alternate embodiments of this invention, a film or a layer of phosphor is disposed on front, back or at an intermediate level of the display structure. Alternatively, a color filter may be similarly disposed in place of the phosphor coating. In these alternate embodiments of the present invention, an illuminating light or an ultraviolet light can be provided preferably near the back side of the display. In an another embodiment of the invention, phosphor can be mixed in with the LCD material. As will be readily appreciated by one skilled in the art, a number of variations of this display architecture using a common plane and a combination of features included and devised to improve the performance of liquid crystal displays are possible. For example, the present invention can be employed in various operating modes for a LCD including three basic operating modes: reflective; transflective; and transmissive. Generally, the reflective operating mode utilizes a reflector and does not require a back-light. The most common reflector used is some form of a mirroring device. The transflective operating mode uses a back-light and a reflector, while the transmissive operating mode relies entirely on a back-light for adequate contrast ratio for the displays.